In general, electrochromic devices are referred to as devices that experience a change in color due to an electrochemical redox reaction caused by the application of an electric field, resulting in a change in light transmission characteristics. There are many appliances using such electrochromic devices. One of the most successful appliances includes a rear view mirror for cars, which controls the glitter from the backside of a car in the night, or a smart window capable of automatic control depending on light strengths. When solar radiation is high, a smart window experiences a color change into a darker color tone in order to reduce the amount of solar radiation. On the other hand, a smart window experiences a color change into a lighter tone on cloudy days. Accordingly, smart windows are energy-efficient appliances. Further, continuous attempts are made to apply electrochromic devices to the field of display such as electric bulletin boards or e-books.
Electrochromic materials forming electrochromic devices are classified into reduction-colored materials and oxidation-colored materials. Reduction-colored materials are those colored by the acquisition of electrons and typically include tungsten oxides. On the other hand, oxidation-colored materials are those colored by the loss of electrons and typically include nickel oxides and cobalt oxides. Other electrochromic materials include inorganic metal oxides such as V2O5, Ir(OH)x, NiOxHy, TiO2, MoO3, etc., conductive polymers such as PEDOT (poly-3,4-ethylenedioxythiophene), polypyrrole, polyaniline, polyazulene, polythiophene, polypyridine, polyindole, polycarbazole, polyazine, polyquinone, etc., and organic electrochromic materials such as viologen, anthraquinone, phenocyazine, etc.
The above inorganic metal oxides generate a change in color when lithium ions or hydrogen ions present in an electrolyte are doped into the inorganic metal oxides. On the contrary, as depicted in the following Reaction Scheme 1, conductive polymers, for example, polyaniline shows a light yellow color when it is present in a completely reduced state, while showing a blue color when it is present in an oxidized state. Various colors can be realized depending on the kinds of such conductive polymer.

In addition to the above-mentioned inorganic metal oxides and conductive polymers, organic electrochromic materials include viologen compounds such as 4,4′-dipyridinium salt represented by the following Reaction Scheme 2. A viologen compound has three types of oxidation states, i.e., V2+ (colorless), V+ (blue) and V0 (light yellow), each oxidation state showing a different color:

Meanwhile, U.S. Pat. No. 5,441,827 (Graetzel et al.) discloses a device having high efficiency and high response rate, the device being manufactured by coating an electrochemically active organic viologen compound, as a single layer, onto the surface of a nanoporous thin film electrode obtained by sintering metal oxide nanoparticles. Additionally, the device uses a mixture of a lithium salt with an organic solvent such as γ-butyrolactone and propylene carbonate, as liquid electrolyte. However, the device using an organic solvent-containing liquid electrolyte has disadvantages in that quenching rate is low, residual images are present after quenching and that the organic materials may be decomposed easily during repeated developing/quenching cycles. Moreover, because the device uses an organic solvent-containing liquid electrolyte, it has additional disadvantages in that evaporation and exhaustion of the electrolyte may occur, the electrolyte may leak out from the device to cause an environmentally unfavorable problem, and that formation into thin films and film-shaped products is not allowed. Therefore, intensive research and development into the use of an ionic liquid (IL) as electrolyte for electrochemical devices have been made recently.
U.S. Pat. No. 5,827,602 (V. R. Koch et al.) discloses an ionic liquid electrolyte based on AlCl3-EMICl (aluminum chloride-1-ethyl-3-methylimidazolium chloride) including a strong Lewis acid. The ionic liquid such as AlCl3-EMICl has no vapor pressure, and thus can solve the problem of exhaustion and decomposition of electrolyte. However, it may emit toxic gases when exposed to a small amount of moisture and oxygen. Moreover, the ionic liquid is problematic in that it has high reactivity with organic/inorganic compounds added to the electrolyte in a small amount and that it is easily decomposed at a temperature of 150° C. or higher.
U.S. Pat. No. 6,667,825 (Wen Lu et al.) discloses an electrochromic device that uses a conductive polymer and an ionic liquid such as [BMIM] [BF4] (1-butyl-3-methylimidazolium tetrafluoroborate) containing no Lewis acid, as electrode and electrolyte, respectively.
Japanese Patent Publication No. 2002-99001 discloses an electrochromic device that uses an electrochromic material such as methyl viologen or trimethyl methylferrocene along with an ionic liquid such as N1113TFSI (trimethylpropylammonium trifluoromethanesulfonimide) or EMITFSI (1-ethyl-3-methylimidazolium trifluoromethanesulfonimide as electrolyte.
However, conventional ionic liquids are problematic in that they are expensive, their preparation and purification are difficult, liquid electrolytes may leak out from electrochromic devices, and that formation into thin films and large sized products is not allowed. Meanwhile, Liang and coworkers prepared a room temperature-melting salt from urea and LiTFSI (J. Phys. Chem. B 2001, 105, pp 9966-9969). Additionally, Chen and coworkers developed a eutectic mixture of acetamide and LiTFSI, the eutectic mixture being used alone as liquid electrolyte for lithium secondary batteries (Electrochem. Commun, 2004, 6, pp 28-32). However, when the above liquid electrolyte was used in a Li/MnO2 battery, both the initial battery capacity and battery capacity after the third cycle were significantly dropped (20%). Therefore, the liquid electrolyte is not suitable for practical use in batteries. In general, a drop in initial battery capacity and that in capacity during repeated cycles are related with a solid electrolyte interface (SEI) film formed on the surface of an anode upon the first charge cycle. In the case of the liquid electrolyte according to the prior art, it is thought that decomposition of the eutectic mixture having an electrochemical window ranging from 0.7 to 4.7V causes degradation of the battery quality despite the presence of a compound capable of forming an SEI film. Additionally, electrochemical reactions in a lithium battery occur through the lithium moved by the electrolyte injected between a cathode and anode. However, because the eutectic mixture according to the prior art has relatively high viscosity, both lithium transfer rate and swellability of electrode with the eutectic mixture are low, resulting in degradation of the overall battery quality. Moreover, there is no suggestion that the eutectic mixture may be applied to electrochromic devices relevant to the present invention.